As known from avionics, wings may experience a stall under certain conditions. An active flow control may be used in an attempt to counteract such a stall. The latter uses a pulsed air stream. The pulsed air stream is generated in a fluidic actuator, for example.
Fluidic actuators represent a highly efficient flow control device, which makes it possible to generate a pulsed air stream for flow control without in the process requiring movable mechanical components in the jet diffuser (fluidic actuator). These actuators are based on the principle of enhancing the flow mechanics, wherein a large quantity of air is diverted by a distinctly smaller quantity of control air. This control air must alternately be supplied to the control terminals of the fluid actuator.
There is a distinction between externally controlled fluid actuators and self-induced fluid actuators. In eternally controlled actuators, an external control air supply is used, which is typically actively controlled by way of valves, e.g., solenoid valves. In self-induced actuators, control fluid is removed from and returned to the respective output lines via a respective structurally separate return channel. Given a suitable dimensioning, this makes it possible to generate a self-induced vibration, a so-called fluidic oscillation.
As a general rule, a self-induced fluidic actuator of the aforementioned type comprises a supply line, two outputs, an interaction chamber as well as two so-called feedback loops, which are structurally separated from the rest of the interaction chamber, as well as from the supply lines. The compressed air-supplied air stream from the supply line is present at a lateral wall of the supply chamber, so that the air stream exits from the first output. At the same time, a portion of the air stream penetrates into the input of the allocated first feedback loop (coupling), and after a time delay again exits at the output of the allocated first feedback loop. The output is located near the input of the supply line, and may hence be used to control the air stream. As it exits, the portion of the air stream coupled in the first feedback loop forces the air stream in the direction of the second output. The process then repeats itself in a quasi mirror inverted fashion on the other side. As a consequence, the fluidic actuator alternately provides a pulsed air stream at both outputs. Since there are no mechanical parts, the fluidic actuator is highly efficient in this form.
A self-induced fluidic actuator of the aforementioned type is known from U.S. Pat. No. 4,227,550. This self-induced fluidic actuator comprises feedback loops and outlet lines.
One major problem relative to self-induced fluidic actuators during active flow control (AFC—active fluid control) has to do with balancing the individual components. A distinction can be made between flow control by the fluid actuator and the control stage of the fluid actuator. The return line (feedback loop) routes a portion of the fluid flowing through from the output of the actuator back to the interaction zone, and there causes the actuator to switch over. The outlet line (output control fluid) feeds a pulsed fluid stream to a recipient (e.g., a control port of the active fluid control fluid actuator stage), wherein the signal of the outlet lines is phase-shifted by 180° degrees relative to each other. On the one hand, the restricting effect exerted by flow control on the control stage is of significant importance. If the restricting effect is too high, signal modulation drops until a constant air stream finally exits the outputs, as a result of which the control signal present at the AFC stage is no longer sufficient. On the other hand, the required installation space for the control stage is high, since the latter depends on the length of the return lines (feedback loops) and the feed lines separate from the latter to the control ports of the downstream stage (output control fluid). The required installation space indirectly gives rise to additional problems. One such problem is that the installation space required with only a limited installation space being available, such as in the case of a wing, may only be realized by a highly complex structural design; on the other hand, this also yields an elevated demand for material, which manifests itself in a weight proportion that must not be underestimated.
Other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.